[hevansrv7a]

I was reading the CAFE report on the -6A. They give calculated "thrust horsepower" THP which seems to NOT be speed times drag. Shouldn't it be? I know this is not obvious from what they published. I built a spreadsheet to iterate the values for parasite and induced drag according to the usual rules and my numbers for drag at 180 mph, starting with CAFE's numbers,agree with theirs. When I extend my iterations to the higher speeds that they use for THP I get a significant difference from TAS x 5280 / 60 * total-drag. Since my iterations start at best L/D = 106 mph and still agree at 180 mph, I'm assuming it's not my curves that are the problem. Therefore I'm stumped. A search of the internet seems to say that THP is speed times drag. Any experts care to help out?

I note as a possible area of explanation that the drag polar graph is in CAS but the formulae just below it are necessrily in TAS because altitude is not given. Is the difference between CAS and TAS the problem here? Which is more correct for drag measurements, CAS or TAS?

I am building a spreadsheet which will, I hope, make it easier to evaluate changes to engine, prop and/or airframe and will require only TAS or CAS at MinSink and the values of MinSink, sink at MaxL/D and aircraft weight as initial inputs. It's too ugly to share yet. The first sheet in the book is built to use the CAFE report's numbers to prove it gets the right results. The next page will be my 7A's. I will be pleased to share this when it's getting good answers for the CAFE 6A.

 

[John C]

need a little tweak on the equations

Thrust hp = brake hp X prop efficiency factor about 0.80

HP = TV/550 where V is fps

Vfps = Vmph X 5280/3600 or = V knots X 6076/3600

there are 60x60 seconds in an hour

Use TAS (in fps) and standard density (.002378) together to get q
Or use CAS (in fps) and actual density at altitude to get q

good luck on your spreadsheet

 

[gmcjetpilot]

Are you an EAA member

EAA has some spread sheets associated with past articles in their Sport Aviation Magazine. The EAA web site has the spread sheets avaiable for down load.

There are many rules of thumbs like prop efficiency of 0.80 (which is a good ball park). If you want ball park than the equations are simple. If you want exact than it gets way more complicated. Prop efficiency can vary from 0.60 to 0.90, which is what in facts turns engine HP into Thrust. The prop efficiency is by definition = Actual Thrust (HP) / Shaft HP. HP is just a unit of WORK, power over time. Work can be described as energy over time: HP, Thrust, Watts or Furlongs per fortnight (joke). Most props on RV's work in the 0.75 to 0.85 range so 0.80 is a good ball park but only applies at one flight condition, especially for fixed pitch props. However for cruise its pretty close and may even be conservative for lower RPM's.

1 HP = 33,000 (foot - pounds) Per minute; where (foot - pound) is energy, e.g., raise 1 pound one foot. Energy can take many forms, not just mechanical.

What makes you go crazy is PROP efficiency changes with HP applied and airspeed: Here is a good primer LINK. Of course don't ignore the fact the engine and airframe efficiency are changing. It gets complicated. That's why FLIGHT TEST is really the only way to analyze a PROP. You can't analyze a prop sans airframe and engine. So when some one say A PROP is 90% efficient, it means nothing out of context of engine HP, airframe, airspeed. (There was a RV8 guy on the forum that went through all this and did a heck of a job, explaining how he did it. He availed himself to some aerodynamic software. Ultimately he had to make some assumptions, but he was rigorous about it.) In the "equations" you will see coefficients of power and thrust. These can be theoretically determined with analysis or backed out from flight test. Getting good data from flight test is always a challenge.

Part of the Prop efficiency is inexorably tied with the airframe & engine, they have to match to get best efficiency. A prop that is efficient on one airframe may be terrible on another. It sounds obvious, but this gets lost in all the numbers. It sounds simple but it gets over looked. A prop that is lauded to be FANTASTIC on XYZ airplane or with such-n-such engine, my not be very good on a RV with your engine.

That is why the Hartzell BA and Sensenich props work well, they where made for the RV airframe, not generic. It takes fine tuning to get that extra 2% or 5% extra efficiency.

Generic Fixed pitch prop equations (ignoring HP and air frame):

[ Pitch x RPM x 60 ] / [12 x 5280] = MPH (x Prop efficiency = MPH actual); Prop efficiency e.g., 0.80

Tip speed;

[ Prop Dia (inches) x 3.1416 x RPM ] / [12 x 60 ] = tip speed (feet/sec)
(limit tip speed to 850 ft/sec wood, 950 ft/sec metal)

Note:
Pitch as measure by diff prop manufactures is not always the same or equiv.
Constant speed props have variable pitch so it gets more complicated but in general C/S props operate at higher efficiency over a broader range of applied power and air speeds.

 

[Kevin Horton]

I also can't reconcile the thrust horsepower numbers on page 2 of the RV-6A APR with the drag vs speed curve on page 4. For example, page 2 says 126 thrust horsepower at 199.7 mph TAS at 7079 ft. That is 180 mph CAS. The curve on page 4 says that there would be about 215 lb of drag at 180 mph.

Thrust horsepower is true air speed times drag. 199.7 mph TAS is 290 ft/s. 215 lb times 290 ft/s = 62,350 ft-lb/s. There are 550 ft-lb/s per hp, so this is 62,350/550 = 113 THP, where as page 2 says 126 thrust horsepower.

For the 9058 ft case, I get 97 thrust horsepower, vs 111 hp listed in the APR.

I'm baffled too. Mind you, we've just finished our Friday night bottle of wine, so perhaps the old brain isn't firing on all cylinders.

 

[hevansrv7a]

Explaining my approach and thanks

Kevin is directly on point. Thanks for trying. I take a weird, perverse sense of pride in having gotten that answer even for a minute. I'll bet Kevin will solve the problem when the wine wears off!

George and John, while no doubt correct, are coming at it from the opposite end. I'm trying to develop a tool that does not depend upon assumptions about, for example, prop efficiency. That is why I'm trying to directly determine drag at a knowable point on the curve - minimum sink - and work from there. If HP is force times velocity, (33,000 foot pounds per minute) then thrust HP should be (total)drag times velocity no matter what the prop is doing and no matter how thin or thick the air. If my approach works, it will produce the correct answer without having to use expensive instrumentation. It is already producing the correct answer if best L/D is known and I think I can get it to work directly from minimum sink speed and minimum sink fpm. I'll keep working on it and let you fellows know either of progress or stumbling blocks. Thanks.

 

[elippse]

Tip speed;

[ Prop Dia (inches) x 3.1416 x RPM ] / [12 x 60 ] = tip speed (feet/sec)
(limit tip speed to 850 ft/sec wood, 950 ft/sec metal).

Hey! You left out the aircraft's speed! TS = [(VROT)^2 + (VFWD)^2]^1/2
When I do that I call it a brain f**t!

 

[elippse]

The equations I use in evaluating props on a model of the aircraft say that an RV-6 has its best L/D at about 95 mph IAS at 1440 lb; this is not, necessarily, where the best ROC occurs since prop efficiency vs TAS enters in. A Lancair 235 has its best L/D at about 110-115 mph IAS, but it has a much lower induced loss because of its considerably higher aspect ratio, 7.5:1 vs 4.8:1. Usually as a plane's overall drag goes down its speed for L/D max goes higher. A quick estimate of a fairly-clean airframe's equivalent parasite drag area for a retract is to multiply wing area by 0.018, and for a fixed-gear is about 0.02. For a 110 sq.ft. RV-6 that comes out as 2.20 and for a 75 sq.ft. Lancair it's about 1.35. Don't forget when calculating the induced loss coefficient to put the Oswald Efficiency Factor in the denominator along with AR and pi. Depending on wing-tip shape it will be anywhere between 0.7, rounded-tip, to 0.82, slashed-tip.

 

[hevansrv7a]

for Ellipse

...The equations I use in evaluating props on a model of the aircraft say that an RV-6 has its best L/D at about 95 mph IAS at 1440 lb... Usually as a plane's overall drag goes down its speed for L/D max goes higher. ...

Thanks for getting involved!

My somewhat simplistic understanding is that the shape of the drag polar is identical for all fixed wing airplanes. The parasite drag rises as the square of the velocity and the induced drag declines as the square of the velocity. And the best L/D is where the two are equal. Under what circumstances would an airplane's overall (total) drag go down and the speed for L/D max not go higher? If that were to happen would it not violate the shape of the curves? If the shape is not violated, then for the same HP, only a shift to the right in the L/D Max speed could produce a higher top speed.

CAFE says the best L/D for the RV-6A is 106 mph. My 7A 's best sink speed is about 88 mph and best L/D (1.316 x) is thus about 116 mph. (these numbers could be 3-4 mph high at most). I don't understand why your understanding of a 6 has much lower speeds than CAFE's 6A unless the 210 pound difference explains it; they used 1650 for weight. Since the -6 has a little less drag than a 6A shouldn't yours be a little higher if all else were equal?

Thanks again.